Substantial advances have been made in recent years in the field of color ink jet printing, and many of these advances relate to improving the print quality of the printed media, which include plain paper, special papers and various types of transparencies. Typical of these advancements in this art and technology are the various improvements embodied in the Hewlett Packard "PAINTJET" printer and described in some detail in the Hewlett Packard Journal, Volume 39, No. 4, August 1988, incorporated herein by reference.
In this field of color ink jet printing, various approaches have been used to control the overall pattern, size and spacing of individual ink drops ejected from an ink jet printer (hereinafter "dots") and printed within a given surface area of media in order to control the print quality of the printed media. This control is desirable to insure that the drops printed on one "pass" or traverse of an ink jet printhead relative to adjacent print media have an adequate time to dry before another overlying pass is made within that same printed area.
Inadequate drying time of these printed dots will produce a number of undesirable characteristics in resultant print quality, depending upon the type of print media used. For example, in the field of color ink jet printing on transparencies where ink colors such as cyan, yellow, and magenta have been printed over a given area in dot-on-dot (DOD) fashion, excessive volumes of ink per unit area on these transparencies will cause the ink to bead up, coalesce and sometimes produce banding as a result of an over-saturation of ink on some areas of the printed media.
It is also desirable that dots of different passes and within a given pass dry uniformly and consistently in addition to having adequate time to dry. Non-uniform or inconsistent drying will also produce a number of undesirable print quality effects depending upon the media used. Inadequate drying time is one possible cause of non-uniform drying of the ink, but there are also other causes which are discussed below.
In an effort to avoid the above problems of beading, coalescence and over-saturation using dot-on-dot (DOD) printing, some color ink jet printers have used dot-next-to-dot (DND) printing processes wherein successively printed dots are ejected onto side by side pixels in a given printed area. These pixels may, for example, form quadrants or other sections of a larger or super pixel as is known in the art, and color mixing takes place at the side by side or DND interface boundaries within the super pixel. This DND approach to color ink jet printing is preferable to DOD printing processes where either large ink drop volumes or largely water based inks, or both, are used in printing on plain paper.
One approach to solving some of the above problems with printing on transparencies is disclosed in U.S. Pat. No. 4,748,453 issued to Lin et al and incorporated herein by reference. In this patent, there is disclosed one type of complementary multiple-pass DOD ink jet printing process which was developed to avoid some of the above problems related to beading on transparencies. The Lin et al process of U.S. Pat. No. 4,748,453 successive multiple passes of an ink jet pen relative to the print media in a DOD process wherein a first ink swath is completed by the use of two successive ink passes having complementary dot patterns therein. Thereafter, a second swath is laid down immediately adjacent to the first swath and also by the use of two or more successive ink jet passes having corresponding complementary dot patterns therein.
Whereas the above described Lin et al process has to some extent minimized the problems of hue shift and beading on certain types of transparencies, this process can nevertheless still be characterized by the problem of "banding" at the boundaries between adjacent print swaths. This type of banding occurs frequently when largely water based inks are used on plain paper with accelerated drying. The characteristic of banding is one evidenced by repeated variations in the optical density, hue, reflectance, or anything else which visibly delineates the individual swaths or dot rows which make up a printed area. Banding is most visible in printed areas of solid color where it often appears as a narrow white or dark (over-saturated) line within a printed area. Banding can be caused by a number of things that effect dye placement, such as ink concentration, penetration, reaction on, into or with the print media or within the ink itself. Banding can also be caused by variations in ink surface tension which lead to beading, puddling and coalescence of the ink. These latter characteristics in turn can produce variations in ink drying times and non-uniform drying, and these variations can cause banding.
This characteristic of banding is particularly noticeable if the Lin et al process of U.S. Pat. No. 4,748,453 is used on paper instead of transparencies. For example, using the complementary multiple pass process of Lin et al, there are two different kinds of banding which can occur on plain paper depending on the properties of the ink used. A wettable (low surface tension) ink will rapidly cover the printed area, and in this case, drying is most rapid at the edges of the swath where the dye is concentrated after drying as a result of the edge evaporating water. A non-wettable (high surface tension) ink will tend to bead up in the middle of the printed area, thus causing the edges of the printed area to be of a lighter color than in the middle of the printed area where dye is concentrated in puddles or beaded areas. Thus, either a light band or dark band is possible depending on the ink additives used, and both types of banding are produced by non-uniform drying.
In addition to the above, the Lin et al process also exhibits the characteristic of beading on transparencies if the dots per inch or dot size per se exceeds a given threshold, or if the ink used has a low degree or speed of absorption into the transparent media coating. Thus, the Lin et al process of U.S. Pat. No. 4,748,453 is primarily directed to solving print quality problems associated with transparencies where it has met with limited success. The Lin et al process meets with still other more exaggerated problems such as banding when it is applied to either plain or special papers or when the ink formulation used is largely water based. In addition, the Lin et al process does not utilize super pixeling printing in order to minimize the above problems of banding.
Nozzle direction errors and ink drop volume variations as well as paper motion errors, carriage motion errors and paper shrinkage can also produce banding. In the normal mode of operation, weak nozzles in a printhead or missing nozzles therein can produce banding; and the concentration or lack of dye at the interface boundary between successive scans or swaths can also produce banding as previously indicated.